(1) Field of the Invention
The present invention relates generally to a fuel cell, and more particularly to a fuel cell wherein both the anolyte and the catholyte are liquids that can be utilized and stored externally to the cell stack.
(2) Description of the Prior Art
Direct methanol fuel cells (DMFC) have been under investigation for the past decade. These cells are comprised of a solution phase anolyte (methanol) undergoing oxidation on a catalytic surface at the anode, a membrane, and oxygen undergoing reduction on a catalyst at the cathode. Improvements have been made on power density, efficiency, cell stack, and life.
The electrochemical equations associated with the DMFC are:Anode: CH3OH+H2O→CO2+6H++6e−  (1)Cathode: 4H++4e−+→O2→2H2O  (2)Cell Reaction: 2CH3OH+3O2→2CO2+4H2O  (3)
The proposed mechanism for the oxidation of the methanol (equations 4-6) is described below and occurs as a two-step process, where the methanol (CH3OH) oxidizes to form carbon dioxide (CO2) via a carbon monoxide (CO) intermediate step (equation 5). The carbon monoxide is undesirable as it can poison the catalyst and lower voltages and efficiencies.CH3OH→CO+4H++4e−  (4)CO+H2O→CO2+2H++2e−  (5)CH3OH+H2O→CO2+6H++6e−  (6)
The advantages of DMFC cell stacks are that they are lightweight, compact, can operate at ambient temperatures, and run for thousands of hours. Typical voltages are 0.5V at 100 mA/cm2.
A major disadvantage in addition to CO catalyst poisoning of DMFC cell stacks is the crossover of the methanol across the membrane lowering cell voltages and fuel efficiency. Many studies have been conducted on membranes to minimize or eliminate this fuel crossover.
A further disadvantage is that the aforementioned DMFC requires molecular oxygen, which is difficult to store. Oxygen is stored as either a high-pressure gas or a very low temperature liquid. The critical temperature of oxygen is −118.6° C., and the critical pressure is 49.8 atm.
What is desired is a fuel cell that employs liquid fuels that can be stored externally to the cell stack. In U.S. Pat. No. 6,485,851, inventors Sekharipuram R. Narayanan, Thomas I. Valdez, William Chun, disclose a methanol-water fuel cell that employs a liquid oxidant, hydrogen peroxide.
U.S. Published Application 2004/0072044 A1, having inventors John Rusek and Daniel Prater, bearing serial, No. 10/269,046 teaches a direct hydrogen peroxide fuel cell utilizing a proton donating fuel, where the proton donating fuel is methanol. As shown, the hydrogen peroxide is added to the cathode side of the fuel cell, and methanol is added to the anode side of the fuel cell, producing an electric current and a flow of protons, i.e., proton transfer, through the anode/cathode membrane. Methanol and hydrogen peroxide solutions are added periodically to the anode and cathode sides of the fuel cell, usually by automatic computer control system.